1. Technical Field of the Invention
This invention relates to the purification of propylene oxide. More particularly, this invention relates to a method for the purification of propylene oxide formed by the reaction of a hydroperoxide such as tertiary butyl hydroperoxide with propylene.
In the epoxidation of propylene to form propylene oxide by the molybdenum catalyzed reaction of a hydroperoxide such as tertiary butyl hydroperoxide with propylene, a variety of side products accumulate in the reaction mixture. The reaction side products include oxygenated materials such as methyl formate, acetone, isobutyraldehyde, methanol, isopropyl alcohol, etc. Another group of side products that accumulate in comparatively small amounts, such as amounts within the range of about 0.01 to about 0.1 wt. % (i.e., about 100 to 1,000 ppm), based on propylene oxide, are 6 carbon atom hydrocarbon oligomers or dimers of propylene such as 2-methyl pentane, 4-methyl-1-pentene, etc. However, it is normally desirable to provide a propylene oxide product containing about 0.005 wt. % or less of the propylene dimer impurities. The hydrocarbon dimer impurities have boiling point characteristics similar to the boiling properties of propylene oxide and, therefore, they are removed only with great difficulty.
It has been discovered in accordance with the present invention that a purified propylene oxide product having a propylene dimer content of about 0.005 wt. % or less (e.g., from about 1 to about 50 parts per million) can be obtained by liquid-liquid extraction of an impure propylene oxide feedstock with a co-solvent extractant mixture of a C.sub.8 -C.sub.10 paraffinic hydrocarbon and water.
2. Prior Art
The coproduction of an epoxide such as propylene oxide together with a coproduct such as tertiary butyl alcohol by the reaction of an olefin with a hydroperoxide is disclosed in Kollar U.S. Pat. Nos. 3,350,422 and 3,351,635 which are directed to the catalytic epoxidation of an olefin by reaction with a hydroperoxide such as tertiary butyl hydroperoxide. When the olefin is propylene and the hydroperoxide is tertiary butyl hydroperoxide, the principle reaction products are propylene oxide and tertiary butyl alcohol.
Suitable molybdenum catalysts and method of preparing propylene oxides from tertiary butyl peroxide using the catalyst are disclosed, for example, in Marquis et al. U.S. Pat. No. 4,626,596, Marquis et al. U.S. Pat. No. 4,650,886, Marquis et al. U.S. Pat. No. 4,654,427, Marquis et al. U.S. Pat. No. 4,703,027, and Marquis et al. U.S. Pat. No. 4,845,251.
Among the U.S. patents that have issued directed to olefin epoxidation may be mentioned, for example, Marquis et al. U.S. Pat. No. 4,891,437 wherein a process is disclosed wherein tertiary butyl hydroperoxide is reacted with propylene in the presence of a molybdenum catalyst in a polar medium. Other patents directed to olefin epoxidation include patents such as Sargenti U.S. Pat. No. 3,666,777, Stein et al. U.S. Pat. No. 3,849,451, Russell U.S. Pat. No. 3,418,430, Scheng et al. U.S. Pat. No. 3,434,975, etc.
U.S. Pat. No. 3,909,366 teaches that propylene oxide may be purified with respect to contaminating paraffinic and olefinic hydrocarbons by extractive distillation in the presence of an aromatic hydrocarbon containing from 6 to 12 carbon atoms.
Jubin U.S. Pat. No. 3,464,897 is directed to a method for the separation of contaminating paraffinic and olefinic hydrocarbons such as propylene dimers by azeotropic distillation in the presence of an open chain or cyclic paraffin containing from 8 to 12 carbon atoms. In this regard, see also U.S. Pat. No. 3,607,669.
Schmidt et al. U.S. Pat. No. 3,843,488 also discloses a method for the removal of hexenes, hexanes, methyl pentenes and methyl pentanes from propylene oxide by azeotropic distillation in the presence of an alkane, an alkene or a naphthene containing from 8 to 15 carbon atoms.
Kageyama et al. U.S. Pat. No. 3,838,020 discloses a method for the extractive distillation of propylene oxide in order to remove impurities using an extractive solvent mixture composed of a mixture of a glycol and dioxane, butyl acetate, 2-ethyl hexanol or mixtures thereof.